When oil or other liquid hydrocarbons are mixed with water tiny droplets are created within one continuous liquid phase. This mixture will separate by itself only very slowly or not at all.

To separate such a mixture into different phases or components the physical method of coalescence is used. Essentially the small droplets are brought in contact with each other to form larger droplets. These larger droplets can then be separated more easily, for instance by gravitation due to the different density of water and oil.

Depending on the mixture and the desired separation quality different types of coalescer systems will be used: